Non-woven fibrous webs or media have been manufactured for many years for many end uses including filtration. Such non-woven materials can be made by a variety of procedures including air laid, spun bonding, melt bonding and papermaking techniques. The manufacture of a broadly applicable collection of media with varied applications, properties or performance levels using these manufacturing techniques have required a broad range of compositions of fiber and other components and often require multiple process steps. In order to obtain an array of media that can serve to satisfy the broad range of uses, a large number of compositions and multi step manufacturing techniques have been utilized. These complexities increase costs and reduce flexibility in product offerings. A substantial need exists to reduce complexity in the need for a variety of media compositions and manufacturing procedures. One goal in this technology is to be able to make a range of media using a single or reduced number of source materials and a single or reduced numbers of process steps.
Media have a variety of applications including liquid and air filtration, as well as dust and mist filtration, among other types of filtration. Such media can also be layered into layered media structures. Layered structures can have a stepwise gradient that results from layer to layer changes. Many attempts at forming true continuous gradients in fibrous media have been directed towards filtration applications. However, the disclosed technology of the prior art of these filter media are often layers of single or multiple component webs with varying properties that are simply laid against one another, or stitched or otherwise bonded together during or after formation. Bonding different layers together during or after layer formation does not provide for a useful continuous gradient of properties or materials. A discrete and detectable interface between layers will exist in the finished product. In some applications, it is highly desirable to avoid the increase in flow resistance that is obtained from such interfaces in the formation of a fibrous medium. For example, in airborne or liquid particulate filtration, the interface(s) between layers of the filter element is where trapped particulate and contaminants often build up. Sufficient particle buildup between layers at the interfaces instead of within the filter media can result in shorter filter life.
Other manufacturing methods such as needling and hydro entangling can improve the mixing of layers, but these methods often result in a filter media that typically contains larger pore sizes which result in low removal efficiencies for particles less than 20 microns (g) in diameter. Also, needled and hydroentangled structures are often relatively thick, heavy basis weight materials which limits the amount of media that can be used in a filter.
Wet laid methods, such as papermaking methods, are used to make nonwoven media having a wide range of pore sizes for high efficiency filtration of small particulates, together with acceptable basis weights; additionally, wet laid methods can employ non-melt processable materials such as glass fibers. Continuous wet laid methods employ dilute aqueous slurries of fiber that are dispensed onto a moving wire mesh or other open or porous support structure, followed by draining or suctioning the liquid from the slurry through the support structure to form a nonwoven medium. Because fiber slurries tend to settle without some level of turbulence, papermaking processes typically employ headboxes, which are designed to deliver a uniform flow of slurry to the support structure. Many headboxes have significant internal structures designed to maintain the fiber dispersion and prevent settling, while avoiding undue turbulence that causes nonuniformity in the slurries as they are dispensed onto the support structure. To that end, headbox internal structures include pluralities of passages, dividers, channels, and the like. These internal structures converge or otherwise end at the point where the slurry exits the headbox, and a uniform slurry is deposited in continuous fashion near the upstream edge of the moving support structure through an opening, called a slice. The slice extends over the cross web direction of the machine and provides a singular, uniform flow across the web. Headboxes are designed to pump large quantities of slurry rapidly onto the support in order to meet industrial requirements of rapid media formation.